This invention generally relates to a drop-on-demand inkjet printer having a droplet separator that includes a mechanism for assisting the selective generation of micro droplets of ink.
Many different types of digitally controlled printing systems have been invented, and many types are currently in production. These printing systems use a variety of actuation mechanisms, a variety of marking materials, and a variety of recording media. Examples of digital printing systems in current use include: laser electrophotographic printers; LED electrophotographic printers; DOT matrix impact printers; thermal paper printers; film recorders; thermal wax printers; dye diffusion thermal transfer printers; and inkjet printers. However, at present, such electronic printing systems have not significantly replaced mechanical presses, even though this conventional method requires very expensive set-up and is seldom commercially viable unless a few thousand copies of a particular page are to be printed. Thus, there is a need for improved digitally-controlled printing systems that are able to produce high-quality color images at a high speed and low cost using standard paper.
Inkjet printing is a prominent contender in the digitally controlled electronic printing arena because, e.g., of its non-impact, low-noise characteristics, its use of plain paper, and its avoidance of toner transfers and fixing. Inkjet printing mechanisms can be categorized as either continuous inkjet or drop-on-demand inkjet. Continuous inkjet printing dates back to at least 1929. See U.S. Pat. No. 1,941,001 to Hansell.
Drop-on-demand inkjet printers selectively eject droplets of ink toward a printing media to create an image. Such printers typically include a printhead having an array of nozzles, each of which is supplied with ink. Each of the nozzles communicates with a chamber which can be pressurized in response to an electrical impulse to induce the generation of an ink droplet from the outlet of the nozzle. Many such printers use piezoelectric transducers to create the momentary pressure necessary to generate an ink droplet. Examples of such printers are present in U.S. Pat. Nos. 4,646,106 and 5,739,832.
While such piezoelectric transducers are capable of generating the momentary pressures necessary for useful drop-on-demand printing, they are relatively difficult and expensive to manufacture since the piezoelectric crystals (which are formed from a brittle, ceramic material) must be micro-machined and precision installed behind the very small ink chambers connected to each of the inkjet nozzles of the printer. Additionally, piezoelectric transducers require relatively high voltage, high power electrical pulses to effectively drive them in such printers.
To overcome these shortcomings, drop-on-demand printers utilizing thermally-actuated paddles were developed. Each paddle includes two dissimilar metals and a heating element connected thereto. When an electrical pulse is conducted to the heating element, the difference in the coefficient of expansion between the two dissimilar metals causes them to momentarily curl in much the same action as a bimetallic thermometer, only much quicker. A paddle is attached to the dissimilar metals to convert momentary curling action of these metals into a compressive wave which effectively ejects a droplet of ink out of the nozzle outlet.
Unfortunately, while such thermal paddle transducers overcome the major disadvantages associated with piezoelectric transducers in that they are easier to manufacture and require less electrical power, they do not have the longevity of piezoelectric transducers. Additionally, they do not produce as powerful and sharp a mechanical pulse in the ink, which leads to a lower droplet speed and less accuracy in striking the image media in a desired location. Finally, thermally-actuated paddles work poorly with relatively viscous ink mediums due to their aforementioned lower power characteristics.
Clearly, what is needed is an improved drop-on-demand type printer which utilizes thermally-actuated paddles, but which is capable of ejecting ink droplets at higher speeds and with greater power to enhance printing accuracy, and to render the printer compatible with inks of greater viscosity.
The invention solves all of the aforementioned problems by the provision of a droplet separator that is formed from the combination of a droplet assistor and a droplet initiator. The droplet assistor is coupled to ink in the nozzle and functions to lower the amount of energy necessary for an ink droplet to form and separate from an ink meniscus that extends across a nozzle outlet. The droplet initiator cooperates with the droplet assistor and selectively causes an ink droplet to form and separate from the ink meniscus.
Examples of the droplet assistor include mechanical oscillators coupled to the ink in the nozzle for generating oscillations in the ink sufficient to periodically form a convex ink meniscus across the nozzle, but insufficient to cause ink droplets to separate from the nozzle. In the preferred embodiments, such a mechanical oscillator may be a piezoelectric transducer coupled onto the back substrate of the printhead. The droplet assistor may also include devices that lower the surface tension of the ink forming the meniscus in the nozzle. In the preferred embodiments, such devices include heaters disposed around the nozzle outlet for applying a heat pulse to ink in the nozzle, and surfactant suppliers for supplying a surfactant to ink forming the meniscus. Examples of surfactant suppliers used as a droplet assistor would be a mechanism for injecting a micro slug of surfactant into the nozzle when the formation of an ink droplet is desired, and a surfactant distributor continuously applying a thin surfactant film over the outer surface of the printhead so that surfactant is always in contact with ink in the menisci of the printhead nozzles.
When the droplet assistor is a mechanical oscillator, the droplet initiator may be a thermally-actuated paddle. In addition to the mechanical oscillator, the droplet assistor may also include a heater disposed near the nozzle outlet for applying a heat pulse to heat in the nozzle to lower surface tension therein at a selected time, or a surfactant supplier that lowers surface tension in ink forming the meniscus.
Various other combinations of the aforementioned mechanical oscillators and surface tension reducing devices may also be used to form a droplet separator of the invention. In all cases, the use of a cooperating combination of paddle transducers, mechanical oscillators and/or surface tension reducing devices advantageously increases the speed and accuracy of the separating droplets, increases the longevity of the printer, and renders the printer easier and less expensive to manufacture than prior art printers which exclusively utilize a separate, precision-made piezoelectric transducer in each of the nozzles of the printer.